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Diatoms: the most important algae?

Monthly Archives: December 2014

Alongside paleolimnology, there are many other areas that may use diatoms as a research tool, such as when studying climate change, or eutrophication. Measuring pH is another important usage, and one that I used as an example when discussing why diatoms are used as bioindicators: link.

Climate Change

When assessing climate change, it is important to look at the transitional area between two biomes, or ecotone. These transitional areas will shift over time as the climate changes. That means that the location of organisms, such as diatoms, will also shift, as they migrate to areas that were previously inhospitable. The high Arctic is an important reference region for studying climate change, as it is predicted to be more affected by rising temperatures than other areas of the world.

Diatoms were sampled from around the Cornwallis and Prince of Wales islands in the Canadian Arctic Archipelago. These two islands have different temperatures, as Cornwallis is high Arctic, whilst Prince of Wales island spans the transition between high and mid Arctic. There were different diatom assemblages were found around each of these islands.(Antoniades et al. 2014 – link to paper)

In the future this study could be replicated, and by assessing the types of diatoms present in the water, we will be able to track the moving ecotones and so have a greater understanding of the effects of climate change in that area. Changes that happen within the diatom community are likely to be mirrored in high organisms within the food chain, but it is much easier to assess these effects using a small, fast replicating species that it would be with fish, for example. We can only study climate change as well as the funding allows, so it is important to be as cost-effective as possible when collecting data. The cost effectiveness of diatom sampling was also mentioned in my podcast: link.

The Oxford Dictionary of Zoology defines eutrophication as: “the nutrient enrichment of an aquatic ecosystem, such that productivity ceases to be limited by nutrient availability.” This can cause major problems within an ecosystem, as the extreme success of an algal species is often at the cost of other species; algal blooms can drain the oxygen from a water body, causing the fish and other organisms to die.

Diatoms could be viewed as a ‘universal indicator’, with one metric could be used globally. However, a 2005 US study suggests that diatom metrics are only useful in the geographic area in which the samples were collected. Data was collected from five ‘ecoregions’ to create a diatom metric for measuring eutrophication. The ecoregions were 5 areas found to have similar physical factors and diatom species, this allowed them to test the ‘geographic area’ theory.

Indicator species were selected (see methods of paper for more information), and species lists were put together from the regional analyses. These regional species lists were considerably different from the national lists, suggesting that regional metrics can increase accuracy. These new metrics were also found to provide a more accurate assessment of US rivers than European metrics. (Potapova & Charles 2007 – link to paper)

However, diatoms gained a reputation for being hard to study, partially because of the high powered microscopes needed to do so effectively, and so research went into decline. Friedrich Hustedt, one of the most famous diatomists from 1900-1960, had to work as a high school teacher along side his research in order to support his family, as there was little to no funding available in the field. Hustedt described over 2000 diatom taxa, he also had the largest private diatom collection in the world, which is currently stored at the Afred Wegener Institute in Germany (link).

North America had many people who were interested in,and published papers about, diatoms in this era, but the majority were hobbyists or specialists who were not part of a university. Anther factor that held back the development of diatoms as a research topic, is that many of the papers published at this time contained little new information other than describing new species.

One electron microscopes and high-powered computers became more available in the late 1950’s, research into diatoms began to flourish, and the field continues to expand today. Diatoms: Applications for the Environmental and Earth Sciences states that: “Taxa have been described at a rate of about 400 per year over the past three decades, and this rate appears to be accelerating in recent years.”

Diatoms: Applications for the Environmental and Earth Sciences suggests that the history of diatom research be split into three broad categories:

“The era of systemisation” – 1900-1970. Attempts to simplify the complex world of diatoms, in part to ease sharing information with nom-specialists.

“The era of objectification” – present. Powerful computers allow us to predict and model diatom growth and occurrance, allowing for diatoms to be used as research tools themselves.

Ending on a lighter note, a novel use of diatoms dating back to the Victorian era, is samples of diatoms mounted on slides as art. The diatoms are mounted on a slow-drying glue, and historically were arranged using a pig’s eyelash. Modern diatomist, Klaus Kemp, is one of the last known diatom artists, and some of his arrangements can be found on his website, Microlife Services. Embedded below is a short film (less than 5 minutes) about Klaus Kemp, directed by Matthew Killip. It is a stunning piece of work, and I highly recommend you watch it.

One area of research where diatoms are used is in the field of paleolimnology. Paleolimnology, or the study of old lakes, is an area that is especially useful when assessing problems that have arisen. For example, the acid status of a water body, and the effect of changing pH on the species that inhabit the area.

Diatoms are useful as a bioindicator species for a number of reasons, previously discussed here. The main feature of diatoms that makes them useful for examining the past is their silica cell wall, or frustule. This cell wall does not decompose easily, and so can be found extremely well preserved in sediments of almost every water body. This allows for the diatom species to be identified, even when only the fossilised remains are present.

The Soil & Water Conservation Society of Metro Halifax website has an excellent page with a great deal of information about paleolimnology (linked in references), including many applications and interesting findings. The ‘methodology’ section was of particular interest to me, as it stated that diatoms, and in particular their frustules, are the main biological indicator used in paleolimnology. For more information about frustules, see my previous post on the subject: link.

In order to assess how conditions have changed, a good knowledge of previous conditions is necessary; paleolimnology provides information about the water body’s historic condition. Analogue matching uses historic records of the types of diatoms found in the lake sediments, and compares it with the types of diatoms found in current sediments. Slices of sediment are taken and analysed to identify the layers of the different ecological periods in the water body’s history. Modern lakes that have diatom species most similar to those found in the records, are assumed to be displaying similar conditions. This can provide a useful view as to how the environment used to look. (Simpson et al. 2005)

In conclusion, diatoms are a vital tool for examining the history of our water bodies, and a method that will continue to be relevant and useful as monitoring the environment comes increasingly into focus.

The Oxford Dictionary of Science defines an indicator species as “a plant or animal that is very sensitive to a particular environmental factor, so that its presence (or absence) in an area can provide information about the levels of that factor.”

There are many reasons why diatoms make excellent biological indicators, The Great Lakes Ecological Indicators website has compiled a list of reasons in their paper assessing the current usage of diatoms in order to use diatom metrics when monitoring lakes:

“ubiquitous; occur in virtually any aquatic environment

diverse; can provide a fine-grained assessment of environmental conditions

versatile; sensitive to a variety of stressors, particularly water chemistry

short turnover rate; respond rapidly to changing conditions

more time-integrative than “snapshot” environmental measurements

narrow tolerances and specific optima to environmental conditions”

The paper also mentions that because diatoms are easily preserved in sediments, they are of great use in paleolimnology, a subject which I have also discussed on the blog.

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Figure 1. An example of diatoms viewed at 1000x magnification, taken from my own research. (c) Cara Thompson 2014

The acid status of water bodies globally is a growing field of research, as the focus on climate change also increases. The United Kingdom Acid Waters Monitoring Network is responsible for tracking changes in the acidity of water bodies in the UK, and in 2004 the 15 year report was published in Environmental Pollution (Monteith & Evans 2005) discussing changes in pH, as well as how the communities of diatoms have altered. The data was collected from 22 sites across the UK, and sediment samples were taken from these sites for paleolimnological analysis to determine the historical composition.

The diatom samples were collected annually during the summer months, by scraping rocks that were permanently submerged. The samples were then mounted and identified using a microscope at 1000x magnification (Fig. 1). The report found that the type of diatoms found was largely predictable by the pH of the site (Fig. 2)

Figure 2. An example of the species of diatoms found at different acidity levels.

Published in the same issue of Environmental Pollution was a further discussion (Monteith et al. 2005) of how the organisms in a water body respond to rising pH. The species of diatom found changed over the 15 year study, in line with the changing pH. This suggests that diatoms are a suitable indicator of water pH. As the pH becomes more neutral, the species found tends towards the species identified using paleolimnology, suggesting recovery, however, there is still relatively low species diversity when compared to historic conditions.

In conclusion, as well as a measure of pH, diatoms are a very useful biological indicator for many water quality concerns.

One of the most important physical features of a diatom is its unique cell wall, or frustule. Diatoms have a hard cell wall that is made almost completely from silica, a compound which also occurs in nature as quartz. The frustule is made up of two halves, or valves, that are arranged somewhat like a petri dish, with one half overlapping the other (Fig.1). The larger half that overlaps is older; the two halves are held together by a band known as the girdle.

Diatoms are the producers in their food webs, and as such are eaten by many organisms. Any form of protection that improves survival rates for a prey species can be viewed as effective, and the mechanical protection that the frustule provides is a good example of this. Diatoms are more common than other algae with similar growth rates in algal blooms, because of the lower mortality rate associated with their frustule.

The strength of the frustule was tested in the lab using microscopic glass needles to press on the outer surface at known pressures until the frustule cracked. The frustules were shown to withstand a considerable amount of pressure, and even once the surface of the valve has cracked, more pressure is required before the frustule breaks. This process is similar to how bones fracture before they break. The girdle was shown to bend out of shape under pressure before cracking, if the pressure was removed whilst the girdle band was still intact then it regained its original shape. (Fig. 2)

Figure 2. “Properties of an isolated girdle band. a, Sequence showing strong elastic deformation of a girdle band as a function of increasing force. b, girdle band deformed by a calibrated glass needle.” c, Field Emission Microscopy image of the girdle band comparing deformation with and without force (36 nN). Scale bars, 10 mm. (Hamm et al., 2003)

Two of the zooplankton species that feed on diatoms have been found to have very sharp, silica-edged ‘teeth’; this physical feature is likely to have co-evolved with the diatom’s frustule. If they are not crushed, then the diatoms can survive being eaten, and pass through their predator’s digestive system unharmed. Based on the amount of pressure needed to crack the diatom’s frustule, only a relatively large zooplankton would be able to produce the amount of force needed, and other, smaller, zooplankton have been found to choose other prey.

This suggests that the diatom’s frustule is a vital part of their success, as their mortality rates are improved by the mechanical protection provided. This mechanical protection limits the number of organisms capable of feeding on them.